1. Field of the Invention
The present invention relates generally to data storage systems, and more particularly, to envelope detection in a magnetic data storage system.
2. Background of the Related Art
A typical magnetic data storage system includes one or more data storage disks coaxially mounted on a hub of a spindle motor. The spindle motor rotates the disks at speeds typically on the order of several thousand revolutions-per-minute. Digital information, representing various types of data, is typically written to and read from the data storage disks by one or more transducers, or read/write heads, which are mounted to an actuator and passed over the surface of the rapidly rotating disks. The actuator typically includes one or more outwardly extending arms to which in-line suspensions are attached and onto which one or more air bearing sliders are mounted at a distal end of the suspensions. One or more transducers, in turn, are disposed on the air bearing slider. Airflow produced above the disk surface by the rapidly rotating disks results in the production of an air bearing upon which the aerodynamic slider is supported, thus causing the slider to fly a small distance above the rotating disk surface.
The actuator arms are interleaved into and out of the stack of rotating disks, typically by means of a rotary voice coil assembly mounted to the actuator. The rotary voice coil assembly generally interacts with a permanent magnet structure, and the application of current to the coil in one polarity causes the actuator arms, suspensions and sliders to shift in one radial direction, while current of the opposite polarity shifts the actuator arms and sliders in an opposite radial direction.
In a typical magnetic digital data storage system, digital data is stored in the form of magnetic transitions on a series of concentric, closely spaced tracks comprising the surface of the magnetizable rigid data storage disks. The tracks are generally divided into a plurality of sectors, with each sector comprising a number of information fields. One of the information fields is typically designated for storing data, while other fields contain sector identification, synchronization and radial position information, for example. Data is transferred to and retrieved from specified track and sector locations by the transducers being moved from track to track, typically under the control of a position controller.
The transducer, also referred to as a read/write head, is one of the most important components in a magnetic disk drive system. The transducer assembly typically includes a read element and a write element. A common type of read element is the magnetoresistive (MR) head. A conventional read head operates by sensing the rate of change of magnetic flux transitions stored on the surface of a magnetic disk. The MR head produces an electrical output signal in response to the sensed magnetic flux transitions. The MR head""s output signal is velocity independent.
Writing data to a data storage disk generally involves passing a current through the write element of the transducer assembly to produce magnetic lines of flux which magnetize a specific location of the disk surface. Reading data from a specified disk location is typically accomplished by a MR read element transducer sensing the magnetic field or flux lines emanating from the magnetized locations of the disk. As the read element passes over the rotating disk surface, the interaction between the read element and the emanating field from the magnetized locations on the disk surface results in the production of electrical signals, commonly referred to as readback signals, in the read element.
MR heads represent an important improvement in magnetic disk drive systems. In particular, the output signal of a MR head is not dependent on the relative velocity between the head and the disk. MR heads may employ an inductive write element. In contrast to older head assemblies, a MR head uses a modified read element employing features such as a thin sensing element called an xe2x80x9cMR stripexe2x80x9d. The MR stripe operates based upon the magnetoresistive effect. Namely, the resistance of the MR stripe changes in proportion to the magnetic field of the disk, passing by the MR stripe. If the MR stripe is driven with a constant bias current, the voltage across the MR stripe is proportional to its resistance. Thus, the MR stripe""s voltage represents the magnetic signals encoded on the disk surface. In other arrangements, a constant voltage is applied to the MR stripe, and the resultant current is measured to detect magnetic signals stored on the disk surface.
Although highly beneficial, MR heads are especially susceptible to certain errors. Namely, the resistance of the MR stripe varies in response to heating and cooling of the MR stripe, in addition to the magnetic flux signals encoded on the disk surface. Normally, the MR stripe maintains a steady state temperature as the slider flies over the disk surface, separated by a thin cushion of air created by the rapidly spinning disk. In this state, the stored magnetic flux signals contribute most significantly to the MR stripe""s output signals, as intended. An MR stripe, however, may experience heating under certain conditions, especially when the MR head inadvertently contacts another object on the disk.
Physical contact with the MR head may occur in a number of different ways. For instance, the MR head may contact a raised irregularity in the disk surface, such as a defect in the material of the disk surface or a contaminant such as a particle of dust, debris, etc. Also, the MR head may contact the disk surface during a high shock event, where G-forces momentarily bounce the MR head against the disk surface.
Such physical contact results in heating of the MR head, including the MR stripe. Heating of the MR stripe increases the stripe resistance, which distorts the MR stripe""s output signal. This type of distortion is known in the art as a xe2x80x9cthermal asperity.xe2x80x9d A read channel in a magnetic disk drive, however, requires a reliable readback signal from the MR head, free from irregularities such as thermal asperities. Consequently, severe thermal asperities may prevent the read channel from correctly processing output signals of the MR head, causing a data error.
These data errors may be manifested in a number of different ways. For instance, severe distortions of the readback signal may cause the magnetic disk drive to shut down. Other data errors may simply prevent reading of data on the disk. Such data errors may also prevent writing of data, if the servo signal embedded in the disk cannot be read correctly, or it indicates that the head is too far off track to write data without overwriting data on an adjacent track. This condition is called a xe2x80x9cwrite inhibit errorxe2x80x9d. If data errors of this type persist, the disk drive may deem the entire sector xe2x80x9cbadxe2x80x9d, causing a write inhibit xe2x80x9chardxe2x80x9d error. Repeated thermal asperities may also cause a disk drive to fail a predictive failure analysis measure, falsely signaling an impending disk failure to the disk drive user. As shown by the foregoing, thermal asperities in magnetic disk drive systems may cause significant problems in disk drives that use MR heads.
It is now known that the thermal asperites and other heating/cooling events contribute a thermal signal component (baseline-wander) to the overall readback signal. As such, the readback signal may be understood as a composite signal comprising a magnetic component and the thermal component. A detailed discussion regarding these signal characteristics may be found in U.S. Pat. No. 6,088,176, entitled xe2x80x9cMethod and Apparatus for Separating Magnetic and Thermal Components from an MR Read Signal,xe2x80x9d which is hereby incorporated by reference.
Despite its undesirability, the thermal signal component has been used to advantage in detecting any surface defects on disks. By monitoring the thermal signal component of a readback signal, the foregoing problems related to thermal asperties may be identified and eliminated or mitigated. One attempt to address the effects of thermal asperities is by separating a thermal signal component from the magnetic component. Once separated, the thermal signal component may be analyzed to determine the presence of surface defects on a disk.
However, conventional techniques for detecting a thermal signal in a readback signal have heretofore been unsuccessful in cases of readback signals having strong frequency modulations. Current methods require that a track in question is erased or is magnetically written to with a constant frequency. Such an approach is inconvenient for predictive failure analysis (PFA), since a suspected track pre-written with data would have to first be moved to another track. In addition, the track would be either erased or written at a constant frequency before the thermal signal can be extracted and processed for defects.
A more significant problem arises in the event of a hard data error that cannot be recovered. In general, it is preferable to recover the data from where it was originally written on the disk space. As a result, any attempt to move the compromised data to another track may lead to permanent loss of all or part of the data.
Another reason for monitoring and analyzing readback signals is to identify problems related to head spacing modulations. Head modulation refers to a time varying fly height of the read/write head which may produce hard read errors or write-verify errors. The modulation occurs because of a resonance instability in the head-slider during the read/write-operation. This resonance may be due to airbearing resonance, suspension resonance, slider instability, etc. Head modulation may also be non-periodic which is caused by contact with asperities on the disk surface. The problem manifests itself by causing sinusoidal-like modulations of the readback signal at the airbearing resonance frequency, typically around 200-250 kHz for modern sliders. The head modulations can be present on one track or just a single sector, while the adjacent tracks and sectors are free from modulations.
An illustration of head modulation over one single data sector may be illustrated with reference to FIG. 1. FIG. 1 shows a readback signal 100 contained within an upper envelope 102a and a lower envelope 102b. The sinusoidal modulation of the envelopes 102a-b is clearly visible. This gives rise to data errors in the readback signal 100.
As a result of the problems caused by head modulations, improving the magnetic recording performance in a hard disk drive or a tape drive requires the continuous monitoring of the envelope of the readback signal. An automatic gain control (AGC), for example, uses a signal derived from the envelope of the readback signal to maintain a constant amplitude of the readback signal before detection by the data channel. A void in the magnetic-coating on the disk surface is easily detected as a very low level output of the envelope signal. The time/frequency analysis of magnitude variations in the envelope of the readback signal can, in many cases, reveal problems with the head/disk interference caused by defects on the disk surface, by suspension resonances, by airbearing resonances, by local aerodynamic instabilities of the slider, etc.
Conventional methods and systems for envelope detection include full-wave or half-wave rectification followed by lowpass filtering. However, conventional approaches for envelope detection have proved inadequate. While such methods work well for a readback signal of constant frequency, they are not suited for frequency-modulated readback signals (e.g., readback signals from storage space containing data). Even more difficult is to detect the presence of head modulation over storage space containing data. Detection of head modulation is made difficult because of the large variation in frequencies of the readback signal from the data.
The shortcomings of conventional envelope detection approaches may be illustrated with reference to FIG. 2. FIG. 2 shows two lowpass-filtered, full-wave rectified representations of the upper envelope 102a of the readback signal 100 (shown in FIG. 1). Specifically, a high-frequency bandwidth envelope 202 and a low-frequency envelope bandwidth 204 are shown. The high-frequency bandwidth envelope was the lowpass filtered at 5 MHz, while the low-frequency bandwidth envelope was lowpass filtered at 0.5 MHz. The lowpass filter was a sixth-order, elliptic filter. The sampling rate for this readback data was 500 MHz. Each envelope 202, 204 substantially fails to provide an accurate representation of the upper envelope 102a of the readback signal 100.
Therefore, there exists a need for a system and method for detecting envelope modulation and analyzing readback signals for a thermal signal components and head modulation activity.
In one embodiment, a method comprises receiving a readback signal from a storage medium and determining an unacceptable level of modulation activity of the head assembly based on amplitude characteristics of the readback signal. The method further comprises performing a failure prevention action comprising at least one of: (i) terminating at least one of a read operation and a write operation; and (ii) storing a reference to at least one suspect disk sector to which data is suspected to have been written during the head modulation in the case of a past write operation, or was scheduled to have been written during the head modulation in the case of a future write operation.
Another embodiment provides a method comprising receiving a readback signal from a storage medium, determining modulation activity of the head assembly, preserving data to be written to at least one suspect disk sector suspected of being affected by the modulation activity and storing a reference to the at least one suspect disk sector.